ICs find application in a plethora of application domains. The ICs are typically provided in the form of dies or chips that may be mounted on some carrier such as a printed circuit board, in which the electrical connections between the IC die and the carrier may be provided in any suitable manner, for instance using bond wires, ball grid arrays and so on. In most application domains, the nature of the electrical connections is not particularly critical, as the IC die is usually packaged and protected from the outside world, and the dimensions of the IC die, carrier and electrical connections therebetween exhibit a large degree of design freedom.
IC dies are increasingly becoming multifunctional, for instance IC dies may comprise sensing functionality which may be utilized in a wide variety of different devices. An example is provided in US 2012/0092127 A1 in which a IC die including sensor functionality is included in a mobile phone for providing handwriting and fingerprint recognition. As the IC die is embedded in the mobile phone. In an embodiment in this prior art citation, a flexible upper substrate of a combined sensor device as a platform for direct attachment of one or ICs including but not limited to ASICs is provided. The flexible upper substrate including sensing wires and routing leads may be wrapped around the edge of a lower substrate, with the wrapped around edge carrying one or more ICs, such that the ICs are located in between the lower substrate and the upper substrate, thus facilitating a minimal edge border on the device and the glass cover plate extending to the edge of the mobile phone.
However, in certain application domains the particular nature of the electrical connections between the IC die and its carrier is more critical. For instance, IC dies including ultrasound sensing capabilities are increasingly used as a sensing tip of an ultrasound probe such as an ultrasound catheter, in which the electrical interconnections between the IC die and a body of the ultrasound probe are relatively exposed and therefore more vulnerable to damage.
A non-limiting example of an IC die including ultrasound sensing capability is a capacitive micro-machined ultrasonic transducer (CMUT) device. CMUT devices are increasingly popular because CMUT devices can offer excellent bandwidth and acoustic impedance characteristics, which makes them the preferable over e.g. piezoelectric transducers. Vibration of the CMUT membrane can be triggered by applying pressure (for example using ultrasound) or can be induced electrically. Electrical connection to the CMUT device, often by means of an integrated circuit (IC) such as an application specific integrated circuit (ASIC) facilitates both transmission and reception modes of the device. In reception mode, changes in the membrane position cause changes in electrical capacitance, which can be registered electronically. In transmission mode, applying an electrical signal causes vibration of the membrane.
CMUT devices generally operate with a biasing voltage applied. The CMUT can be operated in so called collapsed mode where the biasing voltage applied is increased above the collapse voltage to restrict the membrane and confine part of it against the substrate. The frequency of operation of the CMUT device is characterised by the material and physical properties of the membrane, such as the stiffness, and the size of the cavity. The bias voltage and application of the CMUT device also influence the operation mode. A pressure causes a deflection of the membrane that is electronically sensed as a change of capacitance. A pressure reading can then be derived.
The electrical interconnections between IC die and the body of the ultrasound probe may be provided using bond wires, but bond wires are relatively fragile and may hamper or even prevent the homogeneous application of a lens material, sometimes referred to as the acoustical window, to the sensing area of the IC die, as care has to be taken that the bond wires are not damaged during this application process. In addition, the height of this lens material is typically governed by the required minimum pitch of the bond wires, which may compromise sensitivity of the probe and may increase the overall dimensions of the cross-section of the probe tip, as the bond wires typically bend around the outer edge(s) of the IC die. This may hamper the formation of compact ultrasound probes, e.g. catheters, for use in environments where such compact probes are a necessity, e.g. cardiac investigations.